Depends how you define going on missions. The first raid on the Pool was on a Monday night, and while the climax was often on weekends, they had to do recon and such up through the week.
And I refuse to believe 13 is a calendar year since they started, since that would be a third of the way through the series.
I think she said that because that's about a year since publication began
My count, with varies breaths between books, puts 13 about 8 months after meeting Elfangor, about Feb 97.